A wide variety of electrical devices have become part of everyday life. In a typical day, a person may use an alarm clock, a computer, a television, and countless other electrically powered tools and appliances. The electric current used to operate each of these devices produces both electric fields and magnetic fields, collectively referred to herein as electromagnetic fields. As current passes through a conductive material, the conductor acts like an antenna, transmitting electromagnetic fields into the surrounding environment much like a radio transmission antenna. Conversely, when a conductor is placed in an electromagnetic field, current is induced in the conductor, much like a radio reception antenna. Consequently, the electromagnetic fields emitted by one electric device may induce electric current, or electromagnetic interference (EMI), in a different electric device. Also, depending on the frequency of the emissions, EMI is sometimes also referred to as radio frequency interference (RFI).
If the amplitude of the EMI is high enough, it may disrupt normal operations of circuitry. In order to ensure that electrical devices may operate in close proximity to one another, various governing bodies, such as the Federal Communications Commission (FCC) in the United States, impose restrictions on EMI and RFI levels.
Computers often include one or more varieties of printed circuit boards, including motherboards, expansion boards, daughter boards, controller boards, network interface cards, and video adapters. Printed circuit boards (PCBs) are relatively thin, layered substrates upon which integrated circuits and other electronic components are attached. A printed circuit board typically includes a plurality of electrically conductive and insulating layers. Conductive layers generally have conductive paths or traces, isolated from one another by insulating material, and routed within a plane. These traces are generally designed to electrically contact conductive portions of the electronic components mounted on the PCB, forming electrical interconnects. Insulating layers electrically isolate these conductive paths from one another. The principle structure of conductive traces and layers of insulating material may also be used on a smaller scale within a packaged microchip having a PCB-like package substrate.
Switching rates and switching current amplitudes are increasing with each new generation of chip design, contributing to increased switching noise at higher frequencies. Power and ground planes consist of conductive layers connected to a power supply and ground, respectively. Noise on the power and ground planes, and particularly switching noise on power planes, may interfere with operation of the integrated circuits connected to the printed circuit board. If power and ground plane noise is too great, it may cause interference in an integrated circuit that result in faulty operation of an electronic device. These types of errors may be intermittent, and therefore, very difficult to diagnose. These intermittent failures may happen more frequently than users or designers realize. Since they are very difficult to detect, it is generally good engineering design practice to minimize the power and ground plane noise that may cause these failures to begin with.
Existing techniques for noise reduction, including the use of multiple low-inductance bypass capacitors, are becoming increasingly less effective at the higher frequencies used by newer integrated circuits. Bypass capacitors are capacitors that have the feature of filtering noise by “short circuiting” the high frequency components of a signal. Also called decoupling capacitors, bypass capacitors are often connected between each power plane and adjacent ground plane to attenuate noise. Low-frequency bypass capacitors typically have values of tens to thousands of microfarads, and suppress low-frequency noise. High-frequency bypass capacitors generally have smaller values and suppress high-frequency noise, such as undesired EMI. Bypass capacitors also provide a momentary charge to compensate when active devices change their current consumption. The charge in the bypass capacitors is replenished from the power supply that is connected between each power plane and ground.
Several of these high-frequency bypass capacitors are often distributed across a printed circuit board. Bypass capacitors store and release charge, which reduces the amplitude of noise on the printed circuit board power and ground plane pairs. Typically, hundreds or even thousands of bypass capacitors may be used on a printed circuit board, passing through the power and ground planes within the board. Bypass capacitors of either the low-pass or the high-pass type intrude on valuable board space, making the board larger, and generally more expensive, than it otherwise would need to be. Thus, reducing the number of bypass capacitors needed for noise reduction may generate board space and cost savings.
Although dielectrics are a desired component of a printed circuit board for isolating adjacent conductive planes, it has been shown, both experimentally and theoretically, that reducing the thickness of the dielectric layer between a power and a ground plane assists in noise reduction. There are several benefits to keeping space between paired power and ground planes at a minimum. When placed closely together, capacitance between power and ground planes is increased. Also, as high-frequency waves propagate between power and ground planes, the electric field is more intense, inducing a stronger current in the conductive layers. The I2R loss in the conductors of the current-carrying planes attenuates the higher frequency noise. Due to skin effect, this attenuation becomes greater at higher frequencies.
As transistor densities increase within integrated circuits, the associated EMI and noise restrictions become increasingly difficult to meet. Various cost-added techniques, such as noise filtering, may reduce or attenuate EMI and noise from printed circuit boards. Large copper planes, however, and ground planes in particular, are very difficult to filter effectively, making power and ground planes significant contributors to EMI and noise. Therefore, a need exists for reducing or attenuating EMI emissions and noise from power and ground planes at higher frequencies without significantly increasing the cost of printed circuit boards.